Default service superclasses and subclasses

Each new database or upgraded database has three predefined default service superclasses: the default user class, the default maintenance class, and the default system class.

You cannot disable or drop any of the default service superclasses.

All of the default service superclasses are created with one default service subclass. You cannot create additional service subclasses for the default service superclasses. The default service subclass is always created with the name SYSDEFAULTSUBCLASS, as follows:

Figure 1. Two-tier service class hierarchy

All work issued by connections to a default service superclass are processed in the default service subclass of that service superclass.

Default service superclasses and their default service subclasses are dropped only when the database is dropped. They cannot be dropped using the DROP SERVICE CLASS statement.

Default user service superclass (SYSDEFAULTUSERCLASS)

By default, all user activities run in the SYSDEFAULTUSERCLASS.

Default maintenance service superclass (SYSDEFAULTMAINTENANCECLASS)

The default maintenance service superclass tracks the internal DB2® connections that perform database maintenance and administration tasks. Connections from the DB2 asynchronous background processing (ABP) agents are mapped to this service superclass. ABP agents are internal agents that perform database maintenance tasks. Asynchronous index cleanup (AIC) is an example of an ABP-driven task. ABP agents automatically reduce their resource consumption and number of subagents when the number of user connections increases on the data server. Utilities that are issued by user connections are mapped using regular service classes. You cannot implement service class thresholds on SYSDEFAULTMAINTENANCECLASS.

The internal connections tracked by the default maintenance service superclass include:

• ABP connections (including AIC)
• Health monitor initiated backup
• Health monitor initiated RUNSTATS
• Health monitor initiated REORG

Default system service superclass (SYSDEFAULTSYSTEMCLASS)

The default system service superclass tracks internal DB2 connections and threads that perform system-level tasks. You cannot define service subclasses for this service superclass, nor can you associate any workloads or work actions with it. In addition, you cannot implement service class thresholds on SYSDEFAULTSYSTEMCLASS. The DB2 threads and connections tracked by the default system service superclass include:

• ABP daemon
• Query Patroller (QP) connections
• Self Tuning Memory Manager (STMM)
• Prefetcher engine dispatchable units (EDUs) (db2pfchr)
• Page cleaner EDUs (db2pclnr)
• Log reader EDUs (db2loggr)
• Log writer EDUs (db2loggw)
• Log file reader EDUs (db2lfr)
• Deadlock detector EDUs (db2dlock)
• Event monitors (db2evm)
• Event monitor fast writers (db2fw)
• Connections performing system level tasks

A Query Patroller connection is an internal connection to the DB2 data server issued by the QP controller (the server component of QP) when QP is started. This connection is established as QP is starting up, and after QP has successfully started, the connection is mapped to the default system service superclass. Whilst QP is starting up, the connection may temporarily be mapped to another service class as part of the normal workload mapping process. During this period, the connection is subject to all controls and thresholds of the service class it is temporarily mapped to.